All patients in both groups will start their examination without any use of sedatives/analgetics. On demand, the patients will then be instructed to use the mask for inhalation of gas (N2O in verum group, O2 in placebo group). In case of insufficient effect of inhalation, patients in both groups will be given midazolam and/or pethidine until satisfactorily relieved from pain or discomfort.